This application claims benefit under 35 U.S.C. xc2xa7119 to German patent application no. 198 50 723.2, which was filed on Nov. 3, 1998, and German patent application no. 198 50 882.3, which was filed on Nov. 4, 1998. These German patent applications are incorporated by reference herein in their entirety.
The invention relates to a film laminate comprising a biaxially oriented polyester film with a base layer at least 80% by weight of which is composed of a thermoplastic polyester, and with an cover layer and a barrier layer located on the cover layer. The invention further relates to the use of the laminate and a process for its production.
Food and drink packaging frequently requires a high level of barrier action (meaning low permeability or low permeation) with respect to gases, water vapor and flavors. A frequently used process for producing packaging of this type comprises treating the plastic films used by metallization, for example, high-vacuum vapor-deposition of aluminum. Another frequently used process involves coating the films with ceramic materials, such as SiOx, AlOx or MgOx. The barrier action with respect to the abovementioned substances is essentially dependent on the nature of the polymers in the film and the quality of the barrier layers applied. For example, metallized biaxially oriented polyester films have very good barrier action with respect to gases, such as oxygen and flavors. Metallized biaxially oriented polypropylene films, in turn, exhibit high water-vapor barriers.
The good barrier properties of metallized or ceramically coated films lead to their use in particular for packaging foods and other consumable items where long storage or transport times pose a risk that the packaged foods may spoil, become rancid or lose flavor if the barrier is inadequate. This applies, for example, to coffee, snacks containing fats (nuts, chips, etc.) and drinks containing carbon dioxide (in pouches).
Coated polyester films are also suitable for thermal insulation of technical devices. For example, aluminum-metallized polyester films can be used to improve insulation in refrigerators. The insulation provided by these films uses the vacuum flask principle and consists essentially of two laminates (a number of layers of aluminum-metallized polyester films) with, for example, sheets of foam welded between them. The sheets of foam, which are welded in under vacuum, server essentially as spacers between the two laminates. To maintain the required vacuum the laminates must exhibit very good oxygen barriers.
When polyester films metallized with aluminum are used as packaging material they are generally a constituent of a film composite (laminate) having two or more layers. Bags produced therefrom can be filled, for example, by a vertical tubular bag forming, filling and sealing machine. The bags are sealed on their inner side (i.e. on the side facing the contents), the sealable layer consisting, for example, of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum layer/adhesive layer/sealable layer. For a laminate thickness of from about 50 to 150 xcexcm the thickness of the metal layer is only from 20 to 50 nm. Even this very thin aluminum layer is sufficient to give adequate protection from light and very good barrier properties.
The oxygen barrier or oxygen permeability is generally measured not on the laminate or on the packaging itself, but on the metallized or ceramically coated polyester film. To ensure good quality of the foods or other consumable items even after relatively long storage times, the oxygen permeability (or permeation) of the metallized film may not be greater than 2 cm3/m2 bar d (no more than 2 cm2 of oxygen per square meter and per day should diffuse through the film on exposure to air at a pressure of 1 bar) and in particular not greater than 1 cm3/m2 bar d. In the future, the demands of the packaging industry will head toward still higher barriers, with attempts to achieve permeability values of less than 1.0 cm3/m2 bar d.
There have been a number of reports dealing with oxygen barriers in connection with aluminum-metallized films (substrates). A detailed summary of the art in this connection can be found, for example, in the dissertation by H. Utz (Munich Technical University, 1995: xe2x80x9cBarriereeigenschaften aluminuiumbed ampfter Kunststoffolienxe2x80x9d [Barrier properties of aluminum-metallized plastic films], the disclosure of which is incorporated by reference herein.
There is as yet insufficient information on the detailed basis of the barrier action in the metaillized film. Important variables are clearly the substrate surface and the nature of the substrate polymer, and also its morphology. It is generally assumed that smooth surfaces give better barrier properties (cf. Utz, pp. 38 et seq.). Weiss et al., in xe2x80x9cThin Solids Filmsxe2x80x9d 204 (1991), p. 203-216, have demonstrated that when various concentrations of titanium dioxide particles are introduced into a coating the resultant oxygen permeabilities after metallization with aluminum increase as the amount of TiO2 rises. According to Utz""s studies there is no direct relationship between the surface. roughness of the PET film and the oxygen barrier.
German patent application no. 19720505.4 proposes a biaxially oriented polyester film with a base layer, at least 80% by weight of which is composed of a thermoplastic polyester, and with a cover layer and a metallic or ceramic layer located on the cover layer, where the cover layer of the film is composed of a polymer or of a copolymer or of a mixture of polymers which comprises at least 40% by weight of ethylene 2,6-naphthalate units, up to 40% by weight of ethylene terephthalate units and, if desired, up to 60% by weight of units of other aliphatic, cycloaliphatic or aromatic diols and/or dicarboxylic acids, with the proviso that the Tg2 value of the polyester film is greater than the Tg2 value of the base layer but smaller than the Tg2 value of the cover layer. The entire disclosure of German patent application no. 19720505.4 is incorporated by reference herein.
It is an object of the present invention to provide a film laminate which exhibits a high oxygen barrier (less than 0.05 cm2 of oxygen per square meter and per day should diffuse through the unlaminated base film on exposure to air at a pressure of 1 bar). The other properties of the film laminate should be at least equivalent to those of known packaging films of this type. The production of the film should also be simple and cost-effective.